Allah made it obligatory upon them that one man should fight ten. This was heavy and difficult for them, so Allah lightened it for them so that one man would fight only two. Concerning this, Allah revealed, "If there are twenty steadfast among you..." [Anfal 8:65] to the end of the verse.Then He said, "Had it not been for a decree from Allah that preceded, you would have been touched for what you took by a great punishment" [Anfal 8:68], meaning the spoils of Badr, that is, were it not that I do not punish whoever disobeys Me until I have given him prior warning.
افترض الله عليهم أن يقاتل الواحد عشرة، فثقل ذلك عليهم، وشق ذلك عليهم، فوضع ذلك عنهم إلى أن يقاتل الواحد رجلين، فأنزل الله في ذلك: {إن يكن منكم عشرون صبرون} إلى آخر الآية، ثم قال: {لولا كتب من الله سبق لمسكم فيما أخذتم عذاب عظيم} [ سورة الأنفال: ٦٨] يعني غنائم بدر، لولا أني لا أعذب من عصاني حتى أتقدم إليه.
No references are available.
Tabari narrated it in his Tafsir (11/263), Tabarani (11/171), and Ibn Hibban (4773) — and this is his wording — all of them via Muhammad bin Ishaq, who said, Abdullah bin Abu Najeeh narrated to us, from Ata bin Abu Rabah, from Abdullah bin Abbas, and he related it. Its chain is hasan because of Muhammad bin Ishaq, for he clearly stated that he narrated directly.